Textural properties and adsorption behavior of Zn–Mg–Al layered double hydroxide upon crystal violet dye removal as a low cost, effective, and recyclable adsorbent

The preparation of adsorbents plays a vital role in the adsorption method. In particular, many adsorbents with high specific surface areas and unique shapes are essential for the adsorption strategy. A Zn–Mg–Al/layer double hydroxide (LDH) was designed in this study using a simple co-precipitation process. Adsorbent based on Zn–Mg–Al/LDH was used to remove crystal violet (CV) from the wastewater. The impacts of the initial dye concentration, pH, and temperature on CV adsorption performance were systematically examined. The adsorbents were analyzed both before and after adsorption using FTIR, XRD, and SEM. The roughness parameters and surface morphologies of the produced LDH were estimated using 3D SEM images. Under the best conditions (dose of adsorbent = 0.07 g and pH = 9), the maximum adsorption capacity has been achieved. Adsorption kinetics studies revealed that the reaction that led to the adsorption of CV dye onto Zn–Mg–Al/LDH was a pseudo-second-order model. Additionally, intraparticle diffusion suggests that Zn–Mg–Al/LDH has a fast diffusion constant for CV molecules (0.251 mg/(g min1/2)). Furthermore, as predicted by the Langmuir model, the maximal Zn–Mg–Al/LDH adsorption capacity of CV was 64.80 mg/g. The CV dimensionless separation factor (RL) onto Zn–Mg–Al/LDH was 0.769, indicating that adsorption was favorable. The effect of temperature was performed at 25, 35, and 45 °C in order to establish the thermodynamic parameters ∆Ho, ∆So, and ∆Go. The computed values indicated exothermic and spontaneous adsorption processes. The study presented here might be used to develop new adsorbents with enhanced adsorption capabilities for the purpose of protecting the water environment.

The generation of wastewater containing dye from numerous industries, including paint, plastics, leather, textiles, pulp and paper, printing, and food industries, is a worldwide problem 1 . The output of dye wastewater has typically increased noticeably in recent years as a result of the rapid expansion of this group of companies. The market today offers more than 100,000 commercial dyes, with annual production exceeding 7 × 10 5 t 2 . Most industrial dyes are toxic to people and other natural organisms because of their non-biodegradability, chemical stability, and carcinogenic and mutagenic potential [3][4][5] . Additionally, these dyes significantly affect the aesthetics of water and block sunlight, which has an impact on the photochemical processes in the marine ecosystem 6,7 .
Organic dyes are regarded as the most harmful water pollutants, even at low levels, due to their high toxicity, carcinogenicity, and lack of biodegradability 8,9 . Crystal violet (CV) is one of these dyes that have attracted a lot of interest due to its discharge into water and potential for contaminating water, in addition to its numerous applications in the textile, printing, leather, and coating industries 10 . Since it is mutagenic, carcinogenic, and non-biodegradable, one of the most harmful dyes is crystal violet 11 . It can also remain in the environment for an extended period. Thus, the treatment of CV dyes from wastewater has grown to be a significant field of study. In recent years, a variety of water decolorization techniques have been developed and are being used to remove the colours from industrial effluents. As a result, many techniques have been tried to remove organic dye from contaminated water, including coagulation 12 , photodegradation 13 , chemical oxidation 14 , flocculation 15 , electrodialysis 16 , membrane filtration 17 , and adsorption 18,19 . Adsorption has generated the most interest among the aforementioned techniques because of its greater adsorption capacity, enhanced removal efficacy, environmental friendliness, and low cost 20,21 . Activated carbon, coal, silica gel, activated alumina, fly ash, and metal oxides are a few of the commonly used adsorbents 22,23 . These adsorbents have drawbacks such as poor adsorbent capacity, which results in secondary contamination, a lack of reusability, and other issues that prevent the widespread use of nanostructured adsorbents 24 . Additionally, anionic clays formed of hydrotalcite or layered double hydroxides (LDH) have become a favorable substitute for removing practically all types of harmful pollutants from wastewater 25 .
LDH solves the problems of conventional adsorbents by providing high removal efficiency, an easy synthesis method, high removal of pollutants, recyclability, and affordability 26 . They have also recently received increased interest for a variety of applications, including catalysis 27 , polymer modification 28 , biomedical treatment 29 , and water treatment 30 . LDHs are effective anion exchangers owing to the existence of replaceable interlayer anions 31 . The following general formula shows LDHs: [M II 1−x M III x (OH) 2 ] x+ . [A n− x/n . mH 2 O], where A n− is an interlayer anion and M II and MI III are the divalent and trivalent metal ions found within the brucite-like layers, respectively 31 . Third metal ions can be added to the binary type of LDH, which can also change the electronic chemical structure and increase electric conductivity, to contribute a large number of active sites with a rapid electron transfer process 32 . Metals such as magnesium, aluminum, and zinc are particularly popular for removing dyes from wastewater [33][34][35] . The zinc element in LDH influences its morphological and electrochemical properties 36 . Several methods are employed to prepare LDH materials with a variety of physicochemical properties, including co-precipitation 37 , ion-exchange methods 38 , hydrothermal methods 39 , urea hydrolysis 40 , ultrasonic irradiation 41 , and rehydration/reconstruction 42 . The co-precipitation process is one of the synthesis processes that are frequently employed in the literature. It is presented as an easy, inexpensive, and quick procedure that can be quickly scaled up for use in industrial settings [43][44][45] . It does so in a way that is good for the environment and produces a nanomaterial of high purity without requiring treatments at high temperatures, high pressure, or toxic organic solvents 46  www.nature.com/scientificreports/ To the best of our knowledge, no previous work has been done to remove crystal violet dye using Zn-Mg-Al/ LDH. Therefore, the design and construction of Zn-Mg-Al/LDH with excellent adsorption properties is still a challenging task. In this study, crystal violet (CV) was removed from contaminated water using a co-precipitation method to create Zn-Mg-Al/LDH. The microstructure and morphology of the produced samples were examined using Fourier transform infrared spectroscopy (FTIR), scan electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray (EDX) spectroscopy. The effects of beginning pH, LDH dosage, initial dye concentration, contact time, and temperature on the removal efficiency of CV dye were investigated using batch research. Isotherm and kinetics models were employed to evaluate the adsorption mechanism and kinetics. The recyclability of Zn-Mg-Al/LDH was examined to determine if the adsorption technique could be made more cost-effective.

Materials and methods
Three different metal nitrates were supplied by LobaChemie, India: magnesium nitrate hexahydrate (Mg (NO 3 ) 2 .6H 2 O), zinc nitrate hexahydrate (Zn (NO 3 ) 2 .6H 2 O), and aluminum nitrate nonahydrate (Al (NO 3 ) 3 .9H 2 O). Crystal violet dye was purchased from Jacquard Products, Canada. Hydrochloric acid and sodium hydroxide were provided by Chemlab NV. The co-precipitation technique was used to prepare the Zn-Mg-Al/LDH. Magnesium nitrate, zinc nitrate, and aluminum nitrate are mixed and dissolved in 100 ml of de-ionized water at a temperature of 80 °C for 4 h with continuous stirring, with a molar ratio of (Zn + Mg): Al = (3:1) and a concentration of Zn:Mg of 0.1 molar ratio. To prevent a fast rise in pH that could lead to the creation of carbonate containing LDH, the precipitate was well mixed, and pH 9 was obtained, by gradually adding NaOH solution (2 mol/L) to the mixture. The mixture was quickly stirred for another 24 h at room temperature (25 °C). The system was then filtered and washed several times with distilled water until the pH equaled 7. The precipitate that formed was then dried for 24 h at 60 °C in a vacuum oven 47 . The Zn-Mg-Al/LDH was then processed to achieve a uniform particle size, as can be demonstrated in Fig. 1.
An X-ray diffractometer (Empyrean X-ray Diffractometer from PANalytical) was used to detect the XRD patterns of LDH before and after adsorption with Cu Kα radiation (λ = 1.5405 Å, 40 kV, 35 mA), using 2θ = 5-80 • ). An FTIR spectrometer (Bruker Vertex 70) was used to detect the FTIR spectra of LDH and LDH/CV nanopowders. A wide range of wavenumbers between 400 and 4000 cm −1 was used to record the FTIR spectra. The morphology of the synthesized LDH and LDH/CV powders was investigated using the SEM (JSM-IT200) after using a sputter coater (JEOL-ION SPUTTER JFC-1100) to deposit gold onto the dispersed powder on a glass slide. The measurements were done at the Central Laboratory for Microanalysis and Nanotechnology, Minia University, Egypt. Using a UV-visible spectrophotometer (Unico instrument-UV2000-USA) to identify and measure the concentration of CV dye still present in polluted water. The UV-Visible absorbance spectra of CV dye at different concentrations in a liquid solution were analyzed using monochromatic electromagnetic radiation in the UV and visible ranges at a wavelength of 585 nm. www.nature.com/scientificreports/ The solution's pH has a considerable effect on the adsorption procedure. The impact of solution pH on dye adsorption was studied while maintaining a constant temperature and a constant concentration of both the dye and the Zn-Mg-Al/LDH adsorbent. Therefore, 0.01 g of Zn-Mg-Al/LDH was put in 25 mL of CV dilution solution (20 ppm) at a specified pH. The values of pH ranging from 3 to 9 were achieved by using 0.1 mol/L NaOH and 0.1 mol/L HCl solutions. Using a composite glass electrode pH meter, the liquids' pH was measured. Filtering was then used to separate the liquid and solid phases. The quantity of CV dye in the liquid solutions was quantified using a visible and ultraviolet spectrophotometer.
The type of surface-active center and the surface's adsorption capacity are both revealed by the pH point of zero charge (pH pzc ) 48 . The adsorbent's surface has a zero point of charge, PZC, whenever the number of positive charges and the number of negative charges are equal. In order to compute the PZC, 50 mL of deionized water and 5 mg of Zn-Mg-Al/LDH were combined, and the pH was varied from 3 to 9. The solutions were then mixed at 300 rpm for 24 h. The flasks' ultimate pH values were determined. The ∆pH values were plotted against the pH i . The PZC value was obtained; this was calculated as the pH equal to zero 49 .
To study the impact of adsorbent dosage, each flask contained 25 mL of CV diluted solution (20 ppm) at pH 9, and the solutions were shaken using a shaker at 300 rpm while the solutions were at room temperature. Different quantities of Zn-Mg-Al/LDH (0.01-0.3 g) were added to each flask. Following that, filtering was used to separate the solid and liquid phases. To investigate the impact of contact time, 25 mL of CV diluted solution (20 ppm) at pH 9 was added to all flasks along with 0.07 g of Zn-Mg-Al/LDH. From 0 to 180 min, at room temperature, a shaker was used to shake the solutions at 300 rpm until equilibrium was established. After a set period, the CV dye solution was filtered to remove the catalyst. The CV dye concentration was then determined using the UV spectrophotometer.
To evaluate the effects of the initial CV concentration, all flasks with initial dye concentrations ranging from 5 to 500 mg/L at pH 9 received 0.07 g of Zn-Mg-Al/LDH. The adsorbents were filtered out of the mixture after the flasks were shaken at 300 rpm at room temperature. The concentrations of residual CV were calculated using the UV spectrophotometer. Under optimal conditions, the impact of temperature on the adsorption procedure was examined. At varying temperatures of 25, 35, and 45 °C, 0.07 g of Zn-Mg-Al/LDH was added to each bottle (25 mL) at pH 9. The bottles were shaken for 120 min at various temperatures. Following the filtration of the Zn-Mg-Al/LDH from the CV solution, using a UV spectrophotometer, the CV concentration was determined.
The reusability of Zn-Mg-Al/LDH is critical for practical applications. If the adsorbent material is recycled, the adsorption process may be more cost-effective and repurposed. In order to process Zn-Mg-Al/LDH, ethanol was used in a number of cycles for both adsorption and desorption, followed by a lot of bi-distilled water for washing. The ethanol washing method was selected owing to its affordability and availability 50 . The cleaned Zn-Mg-Al/LDH after adsorption was dried for 24 h at 60 °C until completely dry. At pH 9, Zn-Mg-Al/LDH is submerged in 20 mg/L of CV solution. Following the adsorption test, the waste was collected, thoroughly cleaned, and then blended for an additional 120 min in a new CV solution (pH 9, 20 mg/L). Five cycles of this procedure were carried out to examine the recyclability of the LDH.
In order to determine the chemical stability of Zn-Mg-Al/LDH, 5 mg of adsorbent was dissolved in 50 mL of HCl or NaOH at pH 9 (because this is the pH at which all experiments have been run), and the mixture was then shaken for 24 h at room temperature. Before measuring the absorbance, the suspension was centrifuged at 3000 rpm for 5 min. The dissolved Zn +2 ions in the solution were then measured using an atomic absorption spectrophotometer (AAS) (model ZEISS-AA55, Germany). A hollow cathode lamp (operated at 20 mA) is used as the radiation source with a 2 nm spectral bandpass 51 .

Results and discussion
XRD analyses. The X-ray diffraction patterns of the samples are shown in Fig Figure 2B also includes the FTIR spectrum of the Zn-Mg-Al/LDH after adsorption, which shows that the peak dramatically shifts towards the higher or lower wavenumbers, which prove that crystal violet is adsorbed onto the Zn-Mg-Al/LDH. After CV adsorption, FTIR peak intensities have decreased, which may be due to the intermolecular interactions between the CV and LDH's solid phase.
SEM analysis. Figure 3A-C display the obtained SEM images of the produced Zn-Mg-Al/LDH, which depicts a plate-like structure and a regularly rough surface that is stacked one on top of the other and has numerous pores. Meanwhile, the surface morphology of the adsorbent changes after the adsorption of CV dye molecule. Figure 3D,E illustrate the surface coverage owing to the adsorption of CV, indicating the good adsorption efficiency of Zn-Mg-Al/LDH. The elemental composition of Zn-Mg-Al/LDH is depicted by the EDX analyses both before and after adsorption, which are presented in Fig. 3F,G, respectively. Weight changes in element atoms after adsorption were also observed, which is clearly because of the dye molecule's adsorption (C 25 H 30 ClN 3 ).
General surface analysis. LDH surface morphology has received a lot of attention due to its ability to show significant characteristics, such as irregularities and heterogeneities, which may have an impact on how the material is used. Contrary to a smooth surface, topographical changes to a surface, such as random roughness, gratings, or isolated bumps, will change the adsorption process. An increase in surface area occurs when the roughness is greatly increased, which is likely to indicate more material has been adsorbed. In principle; roughness can also change the chemical homogeneity of the surface 64 . The topographic SEM image was examined  Figure 4A,B show the representative 3D images of the Zn-Mg-Al/LDH surfaces both before and after adsorption. The analyzed samples' depth histograms and Abbott-Firestone curves are displayed in Fig. 4C,D. The Abbott-Firestone curves show the depth statistical distribution of the sites on the surfaces of the sample, while the histograms provide the distribution density of the sites on the surface 66 . The horizontal axis is computed as a percentage of the total population, whereas the vertical axis refers to the depth. According to this figure, each sample has a distinctive height distribution, and the depth of Zn-Mg-Al/LDH decreases after crystal violet adsorption from 91.14 to 69.93 µm. A schematic illustration of an S-shaped bearing area curve can be found in Fig. 4E,F. The profile's heights are shown on the vertical axis, and the bearing area lengths are shown on the horizontal axis as a percentage of the profile's overall assessment length 67 . Figure 4E,F also show the roughness characteristics that can be inferred from the bearing area curve, including Mr1, Mr2, Rk, Rpk, and Rvk. The core depth is referred to as the parameter Rk, and it specifies the height of the core material. The value of Rk decreases from 10.52 µm (before adsorption) to 9.85 µm (after adsorption). Reduced peak height, or Rpk, is a parameter that refers to the proportion of peaks above the core profile. The value of Rpk after adsorption decreases from 10.65 to 8.98 µm. The reduced valley www.nature.com/scientificreports/ depth, or Rvk parameter, refers to the portion of deep valleys that extend into the material beneath the core profile. The value of Rvk decreases from 21.82 µm (before adsorption) to 16.68 µm (after adsorption). The adsorption of dye molecules by LDH causes surface covering, which reduces all roughness parameters. The material ratio at the boundary between protruding peaks and the core is indicated by the parameter Mr1. The material ratio at the transition of the deep valleys and the core is indicated by the parameter Mr2. Cartesian graphs were used to evaluate the samples' surface texture directions. Three preferred angles exist, as depicted in Fig. 5, which might be connected to the surface's inhomogeneity 66 . When the aspect ratio of the texture Str is almost 0, the surface is anisotropic; when Str is nearly 1, the surface is isotropic 68 . In this study, Str before and after adsorption was 0.718 and 0.044, respectively, indicating that the Zn-Mg-Al/LDH surface texture is isotropic. The value of Str is decreased due to loading material on the LDH surface. The parameters    www.nature.com/scientificreports/ The roughness skewness Rsk, used to evaluate the symmetry of a surface's variations around the mean plane. The Rsk discusses the features of nonconventional processes, such as porosity and load-carrying ability. Among the Rsk value of 0.237, Zn-Mg-Al/LDH has a peaky surface. Meanwhile, after adsorption, the Zn-Mg-Al/LDH exhibits an Rsk of − 0.242, indicating that the surface of the compound is a valley surface 69 . The roughness kurtosis Rku is applied to calculate the distribution of the peaks above and below the mean plane. For spiky surfaces, Rku > 3; for bumpy surfaces, Rku < 3; perfectly random surfaces have kurtosis = 3 68 . Zn-Mg-Al/LDH has the Rku of 2.740, demonstrating that the surface of Zn-Mg-Al/LDH is a bumpy surface. Meanwhile, Zn-Mg-Al/ LDH after adsorption has the Rku of 4.422, demonstrating that the Zn-Mg-Al/LDH/CV surface is spiky. Table 1 also includes a summary of the fractal dimensions Df and correlation coefficient (R 2 ) calculated for the sample surfaces using the enclosing box method. The R 2 of the linear fit was equal to 0.996 and 0.998 for Zn-Mg-Al/ LDH and Zn-Mg-Al/LDH/CV, respectively; this demonstrates that the data were fit by linear functions very well. The Df is related to the system's complexity 70 . The Df of Zn-Mg-Al/LDH and Zn-Mg-Al/LDH/CV are 1.350 and 1.344, respectively. Adsorbent chemical stability. AAS is used to calculate the zinc concentration in solution at pH 9. Zinc plays an important role in the physiological and metabolic processes of many different organisms and is one of the crucial trace elements; however, higher zinc amounts can be hazardous to the organism 71 . Zinc is one of the heavier metals that pose a threat, so it is important to check its chemical stability and make sure it does not decompose. The concentration of zinc in the water samples was 0.058 mg/L and was observed to be below the permissible limits (5 mg/L) as given by the World Health Organization (WHO) and the National Drinking Water Quality Standard (NDWQS) 72,73 . Zn-Mg-Al/LDH exhibits good chemical stability in simulated water.

Influence of varied factors on adsorption of CV dye.
The effectiveness of the adsorption process was assessed using the dye removal efficiency RE (%) and adsorption capacity q t (mg/g) (mg of adsorbate/g of adsorbent) as Here, C o and C t are concentrations of dye at time 0 and t, respectively (mg/L), m is the adsorbent's weight (g), and V is the solution's volume (L) 74,75 .
Influence of initial pH. The pH of the solution has an impact on both the surface charge of the adsorbent and the degree of adsorption of the dye molecule; this significantly affects both the dye's removal from the aqueous solution and its adsorption capacity. At an acidic pH, H + ions and positively charged dye molecules competed for the adsorption sites on the catalyst's surface. At a higher pH, the electrostatic interaction between the negatively charged surface and the cationic dye accelerates the adsorption process, which results in the surface groups becoming deprotonated 76,77 . The point of zero charge (pH ZPC ) is the pH level of the solution necessary to produce a net zero charge on the surface of the adsorbent. The Zn-Mg-Al/LDH point of zero charge is depicted in Fig. 6. From this plot, ΔpH was 0 at an initial pH value equal to 7.76 (pH ZPC = 7.76). Therefore, when the solution's pH i is greater than 7.76, the Zn-Mg-Al/LDH surface will be negatively charged and be able to attract the cationic dye (CV) via electrostatic interaction. Figure 7A demonstrates the decrease in removal efficiency of CV dye at low pH; at pH 3, the adsorption of CV decreased to 42.93%, which is probably affected by electrostatic repulsion. The removal efficiency sharply increased to 75.81% at pH 9. These data show that variations in pH values have a significant effect on the adsorption of CV as a result of the surface charge changing from positive to negative regions, which enhances the adsorptive process.
Influence of adsorbent amount. The adsorbent dose was also simple to regulate during the wastewater treatment process to determine the effectiveness of the adsorbent. Per 25 mL of CV diluted solution at pH 9, an adsorbent dose ranging from 0.01 to 0.3 g was used to measure its effect. Figure 7B depicts the CV removal efficiency results. LDH removal efficiency rose from 75.81 to 86.39% with an additional increase in adsorbent dosage up to 0.01 g. As the adsorbent amount increases, the number of active sites increases, making them more available during adsorption. Whenever the dosage is raised to 0.07 g, the LDH layers bend to aggregate, decreasing the adsorbent surface area and obscuring the active sites 78 . At the dosage of 0.3 g, the removal efficiency decreased to 40.86%.
Influence of contact time. Using Zn-Mg-Al/LDH, the impact of contact time on dye removal from an aqueous solution was examined, as demonstrated in Fig. 7C. At early stages, Zn-Mg-Al/LDH was shown to have a fast-rising adsorptivity. For a period of 0 to 100 min, the initial rate of CV adsorption was high and quick. The greater number of activation sites that the adsorbent initially possessed to adsorb CV dye either on the surface or in the interlayer position causes a higher rate of diffusion of CV dye to a solid surface 79 . After 100 min, there was a slight slowdown in the rate of adsorption of the CV dye due to the decreasing CV concentration gradient and the lack of active sites in the adsorbent. At 120 min, the adsorption capacity finally reached saturation. www.nature.com/scientificreports/ Influence of initial dye concentration. The adsorption capacity is also influenced by the initial concentration of adsorbate, due to its connection to access sites on the adsorbent surface. As the initial concentration rises, the rate at which the adsorption sites become saturated increases. When the concentration is increased, the high-driving forces that are capable of mass transfer also increase the adsorption capacity. The adsorptivity of adsorption is influenced by the amount of CV dye, and its concentration is an important factor in determining the maximum removal concentration at the optimum dosage, pH, and contact time. The concentration range of CV from 5 to 500 mg/L has been selected for this study, as shown in Fig. 7D. This figure shows that as CV concentrations rose from 5 to 500 mg/L, the removal efficiency gradually dropped from 95.3 to 31.84%. This is due to the availability of adsorption sites at the LDH surface at lower concentrations. As the initial concentration was raised to the constant adsorbent dosage (0.07 g), more dye molecules established themselves and filled all the available active sites 80 . At a high CV concentration, there is a lack of free active sites, so dye molecules remain in the solution, which results in a decrease in dye removal. Because of its relationship with available sites on the LDH surface, the initial dye concentration influences adsorptivity. As the CV concentration rises, stronger driving forces become available for mass transfer, increasing the adsorption capacity as a result 81 . Figure 7E illustrates the influence of the initial concentration of CV dye on adsorption capacity. With the increase in the CV concentrations from 5 to 500 mg/L, the adsorption capacity increases from 1.23 to 53.76 mg/g.
Influence of recyclability. The experiment examined the regeneration of the Zn-Mg-Al/LDH using an ethanol solution to establish that the adsorbent is a good material for industrial applications. This solution functioned as the adsorbent for primary regeneration after repeated washing. The ability of Zn-Mg-Al/LDH regeneration was investigated at 25 °C and pH = 9 when the initial concentration of crystal violet was 20 mg/L. Figure 7F demonstrated that the removal efficiency of Zn-Mg-Al/LDH was reduced from 87.39 to 64.08% after five regeneration cycles, demonstrating the effective regeneration capabilities of the prepared LDH. It is obvious that the reduction in adsorption capacity causes the adsorption efficiency to decrease with an increase in cycle numbers. After the fifth cycle of usage, adsorption performance could be impacted by changes in the adsorbent's chemistry and structure, as well as by changes in the mass transport conditions 82 .
Adsorption isotherm models. The isotherm is a key aspect for actually describing the adsorbate-adsorbent interaction. At a given temperature, the dynamic equilibrium relation between the concentration of the adsorbate and adsorption capacity is demonstrated by the isotherm curve. The Langmuir and Freundlich models, two well-known isotherms, were employed to simulate the solid-liquid adsorption procedure. According to the Langmuir isotherm model, the adsorption process takes place in a monolayer on a homogenous surface 83 .
The Langmuir adsorption isotherm model's non-linear form can be expressed as Here, q e (mg/g) and C e (mg/L) are the adsorption capacity and the equilibrium concentration of the adsorbate, respectively. q max (mg/g) is the maximum adsorption capacity, while K L with(L/mg) is the Langmuir constant (indicates the affinity of the adsorbate for the active sites). The nature of the adsorption procedure can be predicted by the dimensionless separation factor (R L ) as (3) q e = q max K L C e 1 + K L C e www.nature.com/scientificreports/ When R L = 0 suggests that the process of adsorption is irreversible. Adsorption moves forward favorably at 0 < R L < 1, while becoming unfavorable at R L > 1. The adsorption process is demonstrated to have a linear relationship when R L = 1 84 .
The Freundlich isotherm describes a heterogeneous surface with non-ideal and reversible adsorption on active sites with an exponential energy distribution 85 . The Freundlich adsorption isotherm could be represented in its non-linearized form as www.nature.com/scientificreports/ where n is the intensity factor, and K F (mg/g) (L/mg) 1/n is the Freundlich constant for the adsorption capacity. The value of n indicates whether the adsorption is difficult (n < 1), partially difficult (1 ≤ n < 2), or easy (2 ≤ n < 10) 86 .
Using two isothermal non-linear adsorption models, Fig. 8A shows the fit of the experimental data to the equilibrium isothermal Langmuir and Freundlich models. In comparison to linear regression, the non-linear regression approach was found to be superior for obtaining the isotherm parameters and selecting the ideal isotherm 87 . Table 2 summarizes the parameters of the two different models. The Langmuir and Freundlich isotherm models of CV adsorption onto Zn-Mg-Al/LDH adsorbent were obtained to have correlation coefficients (R 2 ) of 0.996 and 0.983, respectively, indicating that the Langmuir and the Freundlich isotherm models provide a more complete explanation for the adsorption of CV dye onto the Zn-Mg-Al/LDH adsorbent.
The separation factor (R L ) provides a comprehensive explanation for the Langmuir equilibrium data. It was determined that CV was adsorbed onto Zn-Mg-Al/LDH, and the R L value (0.769) was less than 1 and more (5) q e = K f .C e 1/n . www.nature.com/scientificreports/ than 0, demonstrating that that adsorption was favorable. The strong adsorption of the adsorbents towards CV molecules is shown by the low R L value 76 . According to Table 2, the Zn-Mg-Al/LDH maximum adsorption value for CV was 64.80 (mg/g). The system fitted well to the Freundlich isotherm with a (1/n) of 0.460 mg/L, which described the favorable adsorption conditions and the degree of LDH surface heterogeneity.
Adsorption kinetic models. To estimate the adsorption mechanism, kinetic and isotherm models can be analyzed in both linear and non-linear modes 88 . To minimize errors, the experimental results were examined using kinetic and isotherm models based on non-linear ways to investigate the adsorption's controlling processes 89 . By fitting the experimental data to the Weber-Morris intraparticle diffusion model (IPD), the Lagergren pseudo-first-order kinetic model (PFO), and the pseudo-second-order kinetic model (PSO) 90 . The PFO model is useful for demonstrating a mathematical correlation between the adsorption rate and the adsorbed mass as follows: The PFO adsorption rate constant is K 1 (min −1 ), and q e and q t (mg/g) represent the amount of solute adsorbed at equilibrium and at any time t, respectively.
The PSO model predicts that the adsorption process will be controlled by chemisorption, which includes valence forces produced by the sharing or exchanging of electrons between the adsorbed species and the adsorbent 91 . The kinetics of PSO can be expressed as.
Here, K 2 (g/(mg min)) is the PSO adsorption rate constant. The CV adsorption mechanism is depicted in Fig. 8B, employing a variety of kinetic models, including intraparticle diffusion, pseudo-second-order reaction (PSO), and pseudo-first-order reaction (PFO). The R 2 values from the PFO (0.750) and PSO (0.920) models are shown to be comparable in Table 3. Due to this, it can be determined that the adsorption kinetics data that were obtained experimentally may adhere more closely to the PSO model than to the PFO model. Furthermore, the calculated adsorption capacity q calc is 4.87 mg/g, which is very similar to the experimental adsorption capacity q exp which was 4.57 mg/g, as revealed in Table 3. Thus, the better fit of the PSO model may indicate that CV dye adsorption on the Zn-Mg-Al/LDH composite is controlled by chemisorption rather than physisorption.
The Weber-Morris Intraparticle-diffusion (IPD) model can be used to determine the potential rate-controlling step in the dye adsorption process as (7) q t = q 2 e K 2 t 1 + q e K 2 t . www.nature.com/scientificreports/ The intra-particle rate constant is described as K ip (mg/(g min 1/2 )), and the boundary layer thickness is mentioned to C ip (The boundary layer effect increases with increasing C ip values). The likelihood that the solute will diffuse within the adsorbent pores increases with the K ip value. Figure 8C shows how Intraparticle-diffusion works. The dye adsorption onto an adsorbent is said to take place in multiple stages, such as adsorption on the surface of the adsorbent, diffusion within the particles (intercalation), and saturation. CV dye adsorption on Zn-Mg-Al/LDH was controlled by multiple steps in the intraparticle diffusion plot at various time points. The diffusion adsorption stage, represented by the first linear step, is the diffusion adsorption of CV on the Zn-Mg-Al/LDH external surface under the driving force of the solution. Under the effect of the driving force of intramolecular mass transfer, the second step depicts CV's intra-particle diffusion through the pores of Zn-Mg-Al/LDH. The equilibrium adsorption stage, in which LDH achieved adsorption equilibrium as a result of internally optimized coordination, is described in the final linear step 84 . Additionally, the boundary layer thickness C ip and the intraparticle diffusion constant K ip were estimated using the plot's second linear segment 92 and are listed in Table 3.
Influence of temperature and calculation of thermodynamic parameters. The important thermodynamic parameters, such as the enthalpy change (ΔH•), entropy change (ΔS•), and free energy change (ΔG•), can be adjusted to obtain a more accurate result, to establish the possibility of adsorbate-adsorbent interactions as 93 .
T is the solution's absolute temperature (K), R is the universal gas constant (8.314 J/mole K), and K d = q e /C e is the thermodynamic equilibrium constant (L/g). Batch adsorption experiments were also carried out with 0.07 g of the Zn-Mg-Al/LDH composite and 25 mL of the 20 mg/L dye solution at a pH of 9 to assess the thermodynamic parameters. The impact of temperature on Zn-Mg-Al/LDH adsorption efficiency during CV adsorption is shown in Fig. 8D. As the temperature increased from 25 to 45 °C, the adsorptivity decreased from 87.39 to 74.30%, demonstrating that the CV dye's interaction with Zn-Mg-Al/LDH is exothermic in nature 94 . Through hydrogen bonding and attraction forces, the CV dye is adsorbed onto the Zn-Mg-Al/LDH. Therefore, the adsorption efficiency of Zn-Mg-Al/LDH decreases as the temperature rises.
The thermodynamic parameters that are impacted by the temperature dependence of the adsorption process are the Gibbs free energy change ΔG o , enthalpy ΔH o , and entropy ΔS o of adsorption. These parameters were calculated using Eqs. (9) and (10). The slope and offset of the ln K d versus 1/T curve were used to calculate the ΔH• and ΔS• values, as demonstrated in Fig. 8E. The thermodynamic parameters' outcomes are listed in Table 4. Therefore, the spontaneity and viability of the adsorption process are demonstrated by the negative ΔG• values at all of the temperatures tested 95 . When the temperature was raised from 298 to 318 K, the values of ΔG o enhanced from − 2.24 to − 0.085 kJ/mol, proving that at lower temperatures, the dye adsorption process happens more spontaneously 94 . The exothermic nature of CV dye adsorption by the Zn-Mg-Al/LDH is also supported by the negative ΔH• value 96 . A negative ΔS• value shows that the molecules of dye adsorb in a systematic manner at the solid-liquid interface (a reduction in the amount of randomness at the dye molecules' adsorption) 97 .

Possible adsorption mechanism.
For a complete understanding of the process, an explanation of the adsorption mechanism is necessary. The nature of the adsorbate, its structural and functional groups, its surface and textural characteristics, dye diffusion, and the type of interaction between the dye molecules and the LDH are all important aspects of the adsorption process 98 . Zn-Mg-Al/LDH's potential adsorption interactions with CV dye are presented in detail in Fig. 9. The CV dye adsorbent's Zn-Mg-Al/LDH adsorption can be influenced by hydrogen bonding, electrostatic attraction, n-π interaction, π-π interaction, mesoporous filling, and surface diffusion. The hydrogen bond that exists between the nitrogen in the CV dye (H-acceptors) and the hydrogen of the hydroxyl groups (H-donors) is noted 99 . The CV-adsorbed sample's FTIR spectrum shows a sharp drop in the OH group intensity and shifts towards the somewhat higher wavenumbers of 3478.97 cm −1 , indicating proof of the presence of dipole-dipole hydrogen bonding 100 . Through the n-interaction, the OH or oxygen bonds on the surface of Zn-Mg-Al/LDH and the aromatic ring in the CV dye interact 101 . When CV aromatic rings interact non-covalently, the π-π interaction takes place 102 .
(8) q e = K ip √ t + C ip . www.nature.com/scientificreports/ Electrostatic attraction mechanisms are usually explained in terms of the interaction between the cationic CV and sites with negative charges on the Zn-Mg-Al/LDH surface when the pH solution > pH PZC (pH solution > 7.76) 103 . The CV-adsorbed sample's FTIR spectrum shows that the peak around 1384.01 cm −1 shifts towards the wavenumber of 1373.69 cm −1 , demonstrating the electrostatic interaction between the positively charged CV and the negatively charged nitrate ions. The surface properties of Zn-Mg-Al/LDH and the adsorption capability, which is directly proportional to the pore surface area, were correlated with the pore-filling mechanism (intraparticle diffusion) and surface diffusion of Zn-Mg-Al/LDH. The material's high surface areas and pore volumes often encouraged the adsorption of organic pollutants because of the strong pore-filling effect 104 .

Conclusion
Water pollution is currently recognized as a major issue on a global scale, and several initiatives are being taken to mitigate its effects. Additionally, water contains numerous pollutants, such as dyes and heavy chemical elements, among others. To remove CV from a polluted solution, Zn-Mg-Al/LDH was prepared in this study through co-precipitation. The co-precipitation method resulted in the successful production of a Zn-Mg-Al/ LDH nanoparticle with a structure that was thoroughly characterized by XRD, FTIR, and SEM. Zn-Mg-Al/ LDH has the maximum peak to valley height R t value of 25.92 µm, whereas Zn-Mg-Al/LDH has R t value of 15.13 µm following adsorption. After adsorption, the dye molecule and the LDH surface adsorb, which clearly results in a decrease in roughness values. The aqueous solution of CV dye was well absorbed by the synthesized Zn-Mg-Al/LDH nanoparticles, which had an excellent adsorption capacity of 64.80 mg/g. At pH = 9 and an adsorbent dose of 0.07 g, the Zn-Mg-Al/LDH nanoparticles had an adsorptive capacity of 87.3%. The pseudosecond-order kinetic fit for the adsorption kinetics was better than the pseudo-first-order fit, indicating that the chemisorption process dominated the adsorption mechanism. In addition, according to intraparticle diffusion, the CV adsorption process onto Zn-Mg-Al/LDH involves several steps, including diffusion within the particles (intercalation), adsorption onto the adsorbent's active sites (saturation), and adsorption on the surface of the adsorbent. The adsorption isotherm was fit using the Freundlich and Langmuir isotherm models. Both models were better at representing the homogenous multilayer adsorption of CV dye on the Zn-Mg-Al/LDH adsorbent. Thermodynamic parameters indicated that CV adsorption on Zn-Mg-Al/LDH was exothermic and spontaneous. After five cycles, Zn-Mg-Al/LDH's removal efficiency for CV dropped to 64.08%.

Data availability
The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.